Magnetic structure and properties of the vanthoffite mineral Na6Mn(SO4)4

A detailed analysis of the magnetic properties of a vanthoffite-type mineral ${\mathrm{Na}}_{6}\mathrm{Mn}{({\mathrm{SO}}_{4})}_{4}$ based on dc magnetization, low-temperature neutron powder diffraction, and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group $P{2}_{1}/c$, where ${\mathrm{MnO}}_{6}$ octahedra are linked via ${\mathrm{SO}}_{4}$ tetrahedra, forming a two-dimensional (2D) sheet structure in the $bc$ plane of the crystal. This gives rise to superexchange interaction between two ${\text{Mn}}^{2+}$ ions mediated by two nonmagnetic bridging anions (Mn-O-O-Mn) and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the $bc$ plane. The magnetic properties are modeled by numerical calculations using an exact diagonalization technique, which fits the experimental results and also provides the antiferromagnetic ground state of ${\mathrm{Na}}_{6}\mathrm{Mn}{({\mathrm{SO}}_{4})}_{4}$. Both experimental and theoretical calculation reveal a quasi-2D type of magnetic interaction in this polyanionic system, where the dominant antiferromagnetic interaction exists in the plane. The determined collinear antiferromagnetic ground state is consistent with the theoretical predictions for a ${J}_{1}\ensuremath{-}{J}_{2}$ Heisenberg triangular antiferromagnetic model.